add conditionals query

queries/conditionals.scm

@@ -0,0 +1,76 @@
+; Which parts of a template render conditionally, for a tool checking that a
+; variable is defined on every path that reaches a use of it.
+;
+; Django binds an "as" target straight into the context, and a tag that pushes
+; no context leaves that binding visible after its group closes. So
+;
+;     {% if a %}{% now "Y" as n %}{% endif %}{{ n }}
+;
+; renders the year when a is truthy and string_if_invalid ("" by default) when
+; it is not — a silent empty string, not an error. queries/locals.scm cannot
+; see that: a locals query matches a reference to the definition whose scope
+; contains it and has no notion of a path that may not be taken.
+;
+; The halves a tool joins, by grouping matches on the @conditional.group node:
+;
+;   @conditional.group    the whole construct
+;   @conditional.branch   one alternative; at most one of a group's branches
+;                         renders on any given pass
+;   @conditional.default  the branch that renders when no other one does
+;
+; A group is exhaustive iff one of its matches carries a @conditional.default.
+; If it is not, some pass through it binds nothing, so every definition inside
+; is conditional. If it is, a definition escapes only when every branch makes
+; it. Each capture names both halves of the pair, so no ancestor walk is needed
+; and no clause has to be attributed to a group by name.
+;
+; Branches nest, so the same test applies again to a group inside a branch.
+;
+; A branch that is also a @local.scope in queries/locals.scm confines its
+; bindings whether or not it renders; the two queries compose and the scope
+; decides first.
+
+; if/elif are alternatives tested in order; else runs when none matched. None
+; of the three pushes a context, so this is the case the check exists for.
+(if_group
+  [
+    (if_clause)
+    (elif_clause)
+  ] @conditional.branch) @conditional.group
+
+(if_group
+  (else_clause) @conditional.default) @conditional.group
+
+; ifchanged renders its body only when the watched value differs from the
+; previous pass, and its else otherwise. It pushes no context either.
+(ifchanged_group
+  (ifchanged_clause) @conditional.branch) @conditional.group
+
+(ifchanged_group
+  (else_clause) @conditional.default) @conditional.group
+
+; The loop body runs once per item and empty runs when there are none, so the
+; two partition every render. Both are inside ForNode's context.push() — the
+; empty branch included, which is easy to miss — so locals.scm scopes both and
+; nothing escapes either way. Named here because the construct does branch, and
+; because a tool reading only this file should not have to infer that it does.
+(for_group
+  (for_clause) @conditional.branch) @conditional.group
+
+(for_group
+  (empty_clause) @conditional.default) @conditional.group
+
+; A cache body is skipped entirely on a cache hit, and CacheNode pushes no
+; context, so a binding made inside it reaches the rest of the template on the
+; first render and is gone on the next. One branch and no default: never
+; exhaustive, which is the right answer.
+(cache_group
+  (cache_clause) @conditional.branch) @conditional.group
+
+; Not here, deliberately: block and partialdef bodies may also fail to render —
+; a child template overrides the block, and a partial renders only where
+; {% partial %} names it — but both clauses are @local.scope in locals.scm, so
+; nothing they bind escapes to be checked. Bodies that always render once
+; (with, filter, autoescape, spaceless, language, localize, localtime,
+; timezone) are not conditional at all, and comment, verbatim and
+; blocktranslate bodies hold no tags to bind anything.

queries/locals.scm

@@ -11,6 +11,13 @@ variable: (identifier) @local.definition
 (block_clause) @local.scope
 (partialdef_clause) @local.scope
 
+; ForNode renders nodelist_empty inside the same context.push() as the loop
+; body, so "{% empty %}" is part of the loop's scope even though it is a
+; separate clause: "{% for x in ys %}a{% empty %}{% now "Y" as n %}{% endfor %}{{ n }}"
+; is empty. queries/conditionals.scm names both clauses as branches of the
+; same group and relies on this.
+(empty_clause) @local.scope
+
 ; blocktranslate pushes a context for its "with" and "count" bindings. Its
 ; asvar target is not one of them: it is written in the same tag but assigned
 ; outside it, and carries an "asvar" field rather than "variable" so that this